Tiling tub walls

I'm going to be installing a tile backsplash and was wondering how you deal with exhisting power outlets and switch's. Is it as easy as loosening the 2 screws holding the switch to the outer box to allow for the .5 inch or so needed? Also, I'll be using a "tumbled" tile form one of the local HI stores, any tips on grouting with all those little holes and cracks

First time poster here. I am about to have a local company remove the existing drop in shower and add a second shower head in the ceiling, tile the walls and ceilings and install a new shower door. I went to the local tile shop and picked out the tile for the for the shower floor and Threshold. The company that will be doing the work has told me that I can't have a shower door sitting on tile and that it must be marble. They said that they can't guarantee the door will not leak if it is on tile. I really don't want a piece of marble for the thresh hold but the tile I picked out. Is my contractor correct? For some reason I think no because I am sure there are doors installed on tile that do not leak. Also, tile store said it can be done so I bought the tile then the owner said the door installation on tile is not recommended. Help please....